1. Field of the Invention
The present invention relates to a steering control apparatus for turning steered wheels in accordance with rotation of a steering wheel. More particularly, the invention concerns a steering control apparatus wherein a steering shaft connected to the steering wheel is mechanically separated from a turning mechanism for turning the steered wheels and wherein the turning mechanism is controlled an electric control by device.
2. Related Background Art
An example of the steering control apparatus of this type is disclosed, for example, in Japanese Patent Publication No. 6-98931 and the structure thereof is shown in FIG. 6. A steering shaft motor 122 is attached to a steering shaft 121 mounted with a steering wheel 120, so that the steering shaft motor 122 can rotate the steering shaft 121. When a turning shaft motor 130 rotates a turning shaft 132, a rack shaft 134 in mesh with a pinion 131 provided at the tip of the turning shaft is displaced along the axial direction, whereby tires 133 are turned in synchronism with the motion of this rack shaft 134. At this time a turning shaft motor control circuit 141 controls driving of the turning shaft motor 130 so that a turning amount detected by a turning displacement amount sensor 137 may be coincident with a target control amount calculated by a steering displacement amount arithmetic unit 144. At the same time as it, a turning reaction force sensor 138 detects turning reaction force exerted on the turning shaft 132 upon this turning operation and a steering effort sensor 124 detects a steering effort exerted on the steering shaft 121. A steering shaft motor control circuit 140 drives the steering shaft motor 122, based on a deviation between the steering effort and the turning reaction force detected by the two sensors 124, 138.
In this way the control of driving of the steering shaft 121 is carried out based on the deviation between the steering effort detected by the steering effort sensor 124 and the turning reaction force detected by the turning reaction force sensor 138. This turning reaction force is, however, detected as a result of such operation that a steering angle sensor 123 first detects a steering angle, the turning shaft motor 130 rotates the turning shaft 132 according to this steering angle, the rotation of the turning shaft 132 displaces the rack shaft 134 to actually turn the tires 133, and the actual turning of tires generates axial force. Therefore, no turning reaction force exists in the initial stage of steering, and the steering effort is thus reflected in the control of rotation of steering shaft 121 as it is. This results in increasing the rotation amount of steering shaft 121 by the steering shaft motor 122, so that the steering reaction force is small. After a while, the tires 133 are actually turned to generate the axial force (turning reaction force) so as to decrease the deviation between the steering effort and the turning reaction force, whereupon the rotation of steering shaft 121 is restrained, thereby increasing the steering reaction force. There appears a phase shift (a temporal shift) between the steering amount of steering wheel 120 and the steering reaction force as described above. This degraded the steering feeling of driver, so as to raise the incompatible steering feeling.